The present invention relates to color picture tubes having improved inline electron guns, and particularly to an improvement in such guns for obtaining an expanded focus lens for reduced spherical aberration.
An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. In one type of inline electron gun, such a that shown in U.S. Pat. No. 3,873,879, issued to R. H. Hughes on Mar. 25, 1975, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having the bottoms of the members facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In a preferred embodiment, the overall diameter of the electron gun is such that the gun will fit into a 29 mm tube neck. Because of this size requirement, the three focusing lenses are very closely spaced relative to each other thereby providing a severe limitation on focus lens design. It is known in the art that, the larger the focus lens diameter, the less will be the spherical aberration which restricts the focus quality.
In addition to the focus lens diameter, the spacing between focus lens electrode surfaces is important because greater spacing provides a more gentle voltage gradient in the lens which will also reduce spherical aberration. Unfortunately, greater spacing between electrodes beyond a particular limit (typically 1.27 mm) is not permissible because of beam bending from electrostatic charges on the neck glass penetrating into the space between the electrodes, which causes electron beam misconvergence. Therefore, there is a need for further development in design of the main focusing lens electrodes which will provide improved focus lenses having reduced spherical aberration.